Is loop quantum gravity wrong? - physicist explains | Don Lincoln and Lex Fridman

Don Lincoln begins by distinguishing loop quantum gravity from other speculative theories, noting it is better developed and makes actual predictions. Crucially, he clarifies that loop quantum gravity is not a theory of everything — it solely attempts to quantize gravity, treating space as having a smallest indivisible unit, analogous to how water is ultimately composed of discrete molecules. This contrasts with Einstein's general relativity, where space is treated as smooth, continuous, and infinitely divisible.

Lincoln then explains the origins of string theory, which was originally developed as a theory of the strong nuclear force and competed directly with QCD (quantum chromodynamics). String theory ultimately lost that competition, but gained new life when researchers noticed it predicted a zero-mass, spin-2 particle — which can be proven to be the graviton. This unexpected feature transformed string theory into a candidate theory of everything, generating enormous excitement in the theoretical physics community.

Returning to loop quantum gravity, Lincoln recounts how an early version of the theory predicted that the speed of light would vary depending on frequency — higher frequency light traveling at a different speed than lower frequency light, due to interaction with the discrete structure of space. Observations of gamma ray bursters, which emit light across all wavelengths from billions of light years away, showed no such disparity in arrival times, seemingly disproving the theory. However, Carlo Rovelli contacted Lincoln to clarify that loop quantum gravity had already revised and abandoned that prediction, meaning the observation invalidated only the older version of the theory.

Finally, Lincoln discusses the landmark 2017 observation of gravitational waves and light from two merging neutron stars, located 140 million light years away. Both the gravitational waves and the light arrived within 1.7 seconds of each other, providing empirical confirmation that gravity travels at the speed of light — a long-held assumption that was now experimentally proven for the first time.